1) Field of the Invention
The present invention relates to a technology to improve gain and wavelength characteristics of a Raman amplifier for a reliable optical transmission system.
2) Description of the Related Art
One of the main problems in optical relay systems using an optical amplification of a wavelength-multiplexed signal light is the wavelength-dependent gain characteristics in an optical amplifier. To achieve quality transmission characteristics of a signal light, it is required to limit the level of each signal component of a different wavelength within a desired range to suppress signal degradation, such as a degradation of the S/N ratio due to a nonlinear effect of the transmission line and/or amplified spontaneous emission. This necessitates a proper design of the wavelength characteristics of the gain in the optical amplifier, such that the variation in the signal light level of each wavelength component can be suppressed. This kind of improvement in the wavelength characteristics of the gain is explored also for the Raman amplifier that is an effective means for extending a transmission distance. As a result of the study, for example, it is now possible to achieve a broadband Raman amplification over 80 nm in a 1.55 μm waveband transmission system using a silica fiber as a transmission line, by multiplexing plural excitation lights of different wavelengths.
Japanese Patent Laid-Open Publication No. 2001-7768 (Page 8 and 9, FIGS. 21 to 23) discloses a wavelength characteristic control method of the optical transmission power according to the Raman amplification. In this method, the average gain is changed while maintaining an even gain characteristic in the signal wavelength band by appropriately distributing power of the wavelength-multiplexed excitation light. By employing this method, it is possible to achieve a Raman amplifier that has a constant output level of each signal light regardless of the total power level of the input signal light.
However, one of the disadvantages of the Raman amplifier is that the wavelength characteristic of the gain also changes when the average gain of the Raman amplifier changes. FIG. 5 is a graph illustrating an example of the wavelength characteristic of a Raman gain, which is obtained by Raman-amplifying a signal light in the wavelength range of 1570 nm to 1608 nm with a backward excitation lights of 1470 nm and 1500 nm using a single mode fiber (SMF). The horizontal axis represents the wavelength of the signal light (nm) and the vertical axis represents the Raman gain (dB). The Raman gain is a ratio of the signal output level when the backward excitation light is input and the signal output level when the backward excitation light is not input. The Raman gain is also called an on-off gain. Two waveform characteristics of the Raman gain with different average Raman gains are shown In FIG. 5. The dotted-line curve represents a wavelength characteristic of the Raman gain when the average gain is 7.8 dB and the values of the Raman gain are shown on the left vertical axis. The solid curve represents a wavelength characteristic of the Raman gain when the average gain is 3.9 dB and the values of the Raman gain are shown on the right vertical axis.
As shown in FIG. 5, when the Raman gain is changed by changing the power of the excitation light, the wavelength dependency increases as the average gain increases. Because of this, even though the power of the excitation light of plural wavelengths is optimally distributed in the Raman amplifier, it is not possible to change only the average gain without changing the wavelength-dependent gain profile. This is an essential characteristic of the Raman amplification and poses a problem in practical use. Consequently, the power of the excitation light is changed first and then the average gain is changed, so that the output signal level from the Raman amplification always remains constant. However, in practical situations, due to a change in the average gain, the wavelength-dependent Raman gain also changes, thus resulting in a slight loss in the evenness of the signal light level of each wavelength component that constitutes an output signal light.
The use of a gain equalizer is disclosed as a means to suppress the wavelength dependency of the Raman gain. For instance, as a practical design, when the Raman amplifier constitutes an excitation light with fewer number of wavelengths (that is, fewer excitation light sources), the unevenness in the wavelength dependency of the Raman gain from the Raman amplification increases, as compared to the case when more number of excitation wavelengths is employed. This unevenness in the wavelength-dependent gain profile is corrected by using a gain equalizer. However, even when the gain equalizer is employed, if the average gain changes, the wavelength characteristic of the Raman gain does not match with the wavelength characteristic of the gain equalizer and an error in correction occurs. Further, even when the average gain is not explicitly changed, depending on the difference in conditions such as the gain efficiency of the optical fiber that carries out the Raman amplification or the loss occurring along a connection path of the optical fiber, etc., the average gain varies even if the excitation power is the same. Consequently, the wavelength characteristic also changes. This change in the wavelength characteristic does not match with the wavelength characteristic of the gain equalizer, thus causing the error in correction.
Hence, in order to maintain a constant Raman gain, the signal light level is measured both at the time of input and at the time of output from the amplifier, as disclosed in Japanese Patent Laid-Open Publication No. 2001-109025 (Page 5 and 6). Thus, the desired control over the Raman gain is achieved.
However, when the method disclosed in the second patent literature is applied to a Raman amplifier that uses an optical fiber as the amplifying medium, it is necessary to measure the signal level at two places separated by a considerable distance because the signal level is measured both at the time of input into the amplifier as well as at the time of output from the amplifier. Thus the structure of the amplifier becomes complex, because a means to transmit the control signals to distant places has to be incorporated.